The invention relates to a cardiac pacemaker.
Different attempts have been made to construct artificial cardiac pacemakers in which the stimulation frequency is adaptable to the patient's momentary physiological requirements.
Physiological control here means influencing the pacemaker so as to adapt the performance capability of the heart to the momentary requirements, i.e. particularly with respect to its basic rate which is independent of the prior patient specific regulation on the basis of electrical signals derived from the heart that were provided solely to avoid the simultaneous occurrence of stimulated pulses and spontaneous heart action.
Attempts in the past to use such a type of stimulation control over a broad range have failed because usually there is only an unclear relationship between the measured value employed and the stimulation rate which determines the heart output or difficulties exist in finding suitable sensors which can be used for maintenance-free operation over longer periods of time--and particularly for implantation in the human body. Some of the physiological parameters previously used for demand dependent control are the saturation of the blood with oxygen, the blood temperature and the QT interval.
With the previously utilized measured values, it is not possible to set direct patient specific relationships between measured value and stimulated heart frequency since, in the human cardiovascular regulating system, complex functional, time specific dependencies must be considered between the parameters indicating physical stress and the resulting cardiac output. For accurate regulation corresponding to each individual case of stress for the patient, a patient specific adjustment would each time require that a stationary state be reached to permit a defined association of control parameter and heart rate. Since, however, the period of time within which a patient can be stressed is limited in time, inter alia because of the exhaustion of physical reserves--particularly if a cardiac illness exists --the necessary stationary state cannot be attained at all to the required extent. Due to the otherwise complex physiological relationships, regulation of the pacemaker frequency according to the known arrangements is therefore encumbered with considerable deviations in the frequency follow-up range so that physiological adaptation of the stimulation rate is not assured with the desirable reliability.
It is the object of the invention to derive a control algorithm from a physiological value which has a relationship to the heart frequency and thus permits more accurate follow-up of the heart frequency according to the patient's requirements.